This invention relates to numerical control devices (referred to hereinafter as NC devices) for machine tools, by which high precision working e.g., cutting, grinding, abrasive working, of a workpiece may be effected with enhanced quality assurance and working efficiency.
Conventionally, the quality assurance of the form precision, dimension precision, roughness of the finished surfaces, etc., of the works which have undergone cutting, grinding, or abrasive working, has been effected, except for certain exceptional cases, by means of an independent measurement apparatus (e.g., circularity, three-dimension, or roughness measurement apparatus) separate from the machine tool. Thus, in order to accomplish quality assurance for the form precision, dimension precision, roughness of the finished surface, etc., of the work that have undergone cutting, grinding, or abrasive working, the workpiece is first removed from the machine tool and is positioned on an independent measurement apparatus separate from the machine tool, where the measurements of the work are effected. If the measurement data of the work is not within the predetermined tolerances and the work is capable of correctional re-working, the work is again positioned on the machine tool, such that a correction re-working based on the measurement data is effected on the work.
After the re-working, the workpiece is again brought on the measurement apparatus for necessary measurements. The measurement/re-working cycles are repeated until the predetermined precision is obtained.
The reason that measurement apparatus separate from the machine tools are necessary for the quality assurance of the workpiece is as follows. In addition to the fact that the movement precision of the machine tools has hitherto been low compared with the precision needed for the measurements, the vibrations and the thermal deformations present in the machine tools further reduce the precision to make precise measurements impractical. Thus, except for one-dimensional measurements, such as the measurements of the diameter of bearings, which are not affected by the mechanical movements precision of the machine tool, only the measurements of the reference positions or interior diameters utilizing touch probes have been effected, at, however, a low precision.
Thus, according to the conventional measurement method, the quality assurance can be effected only with respect to whether the working error of the work is within the predetermined tolerance. If the work is outside of the tolerance, a skilled operator must take measures therefor with try-and-error method based on his past experiences, and remeasurements are effected after a correctional re-working. (The precisions for one-dimensional lengths are exceptions for this.)
Further, even if the magnitudes of errors are known by means of some ingenuity, the work must be transferred and re-alligned on the machine tool since the measurement apparatus and the machine tool are separate from each other. The re-allignment, however, becomes the more difficult as the required precision increases, such that an extreme skill and a long time are needed therefor.
As described above, the conventional NC machine tools and measurement apparatus are intrinsically incapable of effecting corrections for the working errors for attaining quallity assurance.